Outline

1
Outline
1. Introduction to Reconfigurable
Networks 2. Degrading Effects in
Systems 3. Optical Amplifiers 4. Dispersion
Compensation 5. Polarization Mode
Dispersion 6. Modulation Formats 7. Performance
Monitoring 8. Optical Switching
2
Optical Amplifiers Outline

• Different types of optical amplifier
• C-band EDFA
• LC band EDFA
• Raman Amplifier
• Hybrid Amplifier
• Gain Flattening
• Passive gain equalizer
• Active gain equalizer
• Raman-multiple (broad band) pump
• Channel power equalization
• Difference between gain equalization and channel
  equalization
• Active channel power equalizer
• Commercial products
• Transients effects
• Degradation due to transients effects
• Compensation techniques

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4
Energy Level Diagram of EDFAs
5
Gain Spectrum for Different Values of Inversion
6
Non-Uniform EDFA Gain Accumulation
7
Ultra-Wideband (LC) Silica-Based EDFA
Wideband EDFA
1520 1560 nm
(10 m long)
C
WDM
Coupler
Gain
L
1560 1600 nm
C
L
(200 m long)
(nm)
l
Y. Sun, OAA, 1997
8

Broad Band (LC) EDFA Parallel-Type EDFA
Characteristics
M. Yamada, Electron.Lett., 1997
9
Raman Amplifier Setup
Signal In
Signal Out
Pump In
Pump Out

• Advantages of counter-pumping scheme
• Reduced pump-signal cross talk
• Reduced polarization dependency of gain

10
Raman Gain Spectra
Raman gain peak is shifted 13 THz toward longer
wavelength
C. Fludger, OFC00, FF2
11
Distributed Raman Amplifier
DRA features
Raman pump power
? Low optical noise
? Reduced signal power nonlinear effects
4 dB
8?10Gb/s 40 km SMF/span
L.D. Garrett, OFC00 PD42
12
Wide-Bandwidth Hybrid Amplifier (Raman EDFA)
H. Masuda, PTL, 1999
13
Performance of the Hybrid Amplifier (Raman
EDFA)
Noise Figure
Gain Spectrum
H. Masuda, PTL, 1999
14
Equalization Method Using Customized Broad-Band
Filter

• Broad-Band Customized Filters
• Long Period Grating
• Mach-Zehnder Filter

15
Long-Period Fiber Grating Filter

• Index grating with period 100 mm provides
  coupling between the core and cladding modes.
• Period of grating determines the coupling
  wavelength.
• Strength of grating determines the coupling
  strength.

A. Vengsarkar, Opt. Lett., 1996
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Gain Flattened Raman Amplifier using LPG
? Gain without filter ? Gain with filter ?
Noise figure
Gain (dB) NF(dB)
1510 1520 1530 1540
1550 1560
Wavelength (nm)
Gain and noise figure of the dual wavelength
pumped amplifier
F. Koch et al, OFC, 2000
18
Passive Gain Flattening Techniques

1. Adding Dopant
Advantages
a. No extra components in EDFA
b.
Fine tuned by pump power
Disadvantages
a. High noise figure
b. Each EDFA should be identical in the system
c. No long-haul transmission demonstrated yet

d. Special and complicated fiber handling
2
. Using Filter

Advantages

a. Won't affect noise figure

b. Can flatten several EDFAs periodically

c. Very matured for long-haul transmission
Disadvantages
a. Filter function fixed
b. Need to be fabricated separately
Static (passive) filters cannot respond to the
continual changes caused by dynamic changes in
optical networks
19
All-Fiber Acousto-Optic Tunable Filter
H. Kim et al, OFC, 1997
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Layout of a Seven-stage Device ( 75 x 6 mm
) Complete reconfiguration can take place within
1 ms.
B. J. Offrein et al, PTL 2000
22
B. J. Offrein et al, PTL 2000
23
Raman Amplifier using Broadband Pump Unit
12-channel WDM high power laser diode Unit
Net gain of the Raman Amplifier
Y. Emori et al, Electr. Lett., 1999
24
Multi-Pump Flat-Gain Raman Amplifiers
12-pump wavelength (a) Optimal integral pump
spectrum (b) Optimal input pump spectrum
Gross and net gain profile (solid lines)
V. E. Perlin et al, OFC 2002
25
Difference between Gain Equalization and
Channel Power Equalization
Gain Equalization
Channel Power Equalization
Flatten the Gain Spectrum
Equalize Separate Channels
EDFA Gain
Flattened
Intensity (a.u.)
Intensity (a.u.)
Filter
Transmission
Wavelength
Wavelength
This scheme can also handle
different input channel powers
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Channel Power Equalization Techniques
Speed EDFA transients
ASE suppression
Issues
Polarization independent
Crosstalk
Complexity Cost
Parallel loss element scheme
Serial filter scheme
Main Techniques
Single bulk device
Micro-opto-mechanics
Integrated device
28
Parallel Loss Element Scheme
l
l
l
l
2
3
1
4
l
l
l
l
2
3
1
4
DWDM
DWDM
Tap
Power
Electrically
Tunable
Attenuators
Potential attenuator candidates
ASE noise is reduced
Opto-mechanical attenuator
Acousto-Optic modulator
Fiber Bragg grating
29
Dynamic Power Equalization Using MEMs
Variable
Input
Input
Electrode
Electrode
1
l
o/2
3
l
o/4
0v
30v
Reflectivity
air gap
air gap
l
/4
Air Gap
Plate
0
Silicon
Silicon
l
o/2
3
l
o/4
Grating
10 ?s response speed
Output
Fiber
Dynamic range gt 20 dB
Collimation
Fold
MEMs
Polarization independent
Lenses
Mirrors
Focus
Insertion loss lt 10 dB
Input
Lenses
Fiber
l

Demuxed
device plane
J.E. Ford, PTL 1998
30
Dynamic Channel Power Equalization Using
Micro-Opto-Mechanics
-20
Input
-30
-40
-50
-60
Output
Optical Power (dBm)
-30
-40
Amplified
-50
Flattened
-60
-70
1564
1560
1556
1552
Wavelength (nm)
J.E. Ford, PTL,1998
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Integrated Automatic Channelized Equalizer
Size 1.4?1.1 cm
PMF
SMF
SMF
PMF
SMF
PS
Phase Shifter
Grating
Lens
Grating
Equalizer M-Z interferometer with a
grating-lens-grating cascade in one arm
C.R. Doerr, PTL, 2000
32
Automatic Equalization of 40 Channels with
unequal channel powers
C.R. Doerr, PTL, 2000
33
EDFA Gain Dynamics


EDFA cross saturation causes gain transientsdue
to

• Channel turn-on
• Channel re-routing
• Network reconfiguration
• Link failures

EDFA
InputChannels
OutputChannels
EDFA

DroppedChannels
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EDFA Transient Dynamics
Small
Signal
Saturated
Channel
Region
Region
Dropped
Input
Power
Transient
EDFA Gain
Region
Link Loss
Output
Power
3
dB
Input Signal Power
Time
Deep saturation region
Self regulation
High SNR
35
Frequency Response
Energy Level Diagram
I
4
11/2
3
l
(CW)
t
B

m
1-10
s
23
l
(t)
l
(t)
B

A
Crosstalk (dB)
2
I
4
13/2
980
B
1
nm
1480
1520 1570 nm
t21
Signal
nm
10 ms
I
4
0
15/2
10
100
1K
10K
100K
Signal
A
Modulation Frequency (Hz)
36
Fast Power Transients in EDFA Cascades
(8 channels 4 channels dropped, 4 channels
survived)
5
4 channels
Ove
rshoot peak region
dropped
4
3
Total signal power (dB)
2
4 channels survived
1
Initial power recovery
No Transient
0
-1
-20
20
10
0
6
0
14
0
180
m
Time (
s)
Zyskind, OFC96 PD-31
37
Signal Degradation due to EDFA Transients
Problems with both
surviving
and
added
channels
nonlinearity induced by high power transients
low receiver sensitivity due to poor SNR
Nonlinearity Region
Window of
Operability
Receiver Sensitivity Region

• EDFA Transients Dependencies
• Degree of saturations of EDFA, of A/D
  channels, A/D speed
• Nonlinear effects

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Time Response
1 dB power excursion for surviving channels
10
1.0
4 channels dropped
4 channels survive
7.5
0.75
s)
m
Reciprocal Time (
5.0
0.5
Time (
2.5
0.25
m
s
-1
)
0.0
0.0
0
2
4
6
8
10
12
of EDFAs
Zyskind, OFC96 PD-31
40
Transient Compensation Techniques

• System Requirements
• Speed response time f(1/ of EDFAs)
• 10 EDFAs 1 ms
• 100 EDFAs 100 ns
• Dynamic range
• Residual transients
• Cost and complexity

• Techniques for Transient Compensation
• Adaptive optical attenuator
• Pump control
• Dummy wavelength/Link control

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Transient Compensation Technique
Pump Control
EDF
WDM
Tapped
Power
Pump
Diode

• Stresses pump laser

• Each EDFA requires control

42
Transient Compensation Technique
Pump Control
?Pout (dB)
m
Time (
s)
Srivastava, OAA96 PD-4
43
Transient Compensation Technique
Link Control
WSC
Tap
l
l
l
l
1,...,
n
1,...,
n
Cross
Connect
??i
?c
PD
LD
Control Ckt.
nonlinearity
l
surviving
Large power in control channel can cause
cross-phase modulation in surviving channel.
Possible spectral hole burning.
44
Transient Compensation Technique
Link Control
5
4
3
(dB)
2
out
1
P
D
0
-1
-2
0
200
400
600
800
1000
m
Time (
s)
Srivastava, PTL97
45
Transient Compensation Technique
Gain Clamping
l
l
l
l
1
,...
,
n
1
,...
,
n
l
c
Gain-Clamped
EDFA
PLasing
PSignal
10 - 50
10 - 50
PSignal
Lasing
Signal
Tap
Tap
Attenuator
Isolator
Bandpass
filter

• Gain excursion due to relaxation oscillation
  and spectral hole burning
• Stimulated Brillouin scattering of lasing signal

46
Transient Compensation Technique
Gain Clamping
Jackel, OFC97 TuP4
47
Transients Effect in Raman Amplifiers
Reflected signal ?500
optical power (W)
output
Input signal ?10
Modulation frequency 1 kHz with 50 duty
cycle DCF Length (Raman medium) 13.9 km Pump
Power into DCF 25.8 dBm Single Power 0.3 dBm
time, ?s
The leading-edge output overshoot, lasting for
50 ?s, is followed by a small undershoot, and
then reaches a steady state.
C.-J. Chen, Electron. Lett., 2001
48
Control of Transient Effects in Raman Amplifiers
Transient control by monitoring both the total
output power and the output power for one single
channel, then feedback control on the pump laser.
power monitors
control system
PUMP
Gain fluctuation reduced from 1.2 dB without
transient control to ?0.04 dB with transient
control during dynamic add/drop
Control OFF
Control ON
relative gain fluctuation, dB
time, ?s
C.-J. Chen, et al., Electron. Lett., 2001
49
Summary

• Different kinds of optical amplifiers are
  overviewed. The non-uniform gain distribution
  should be carefully taken care of in both static
  and dynamic systems.
• Gain equalization and power equalization using
  passive or active approaches have been discussed
  and compared.
• Degrading effects in optical amplifiers due to
  fast transients need to be minimized in
  reconfigurable networks. Different techniques
  have been demonstrated.